State of the art non-volatile memory devices are typically constructed by fabricating a field effect transistor in a silicon substrate. The field effect transistor is capable of storing electrical charge either in a separate gate electrode, known as a floating gate, or in a dielectric layer underlying a control gate electrode. Data is stored in a non-volatile memory device by changing the threshold voltage of the field effect transistor through the storage of electrical charge over the channel region of the substrate. For example, in an n-channel enhancement device, an accumulation of electrons in a floating gate electrode, or in a dielectric layer overlying the channel region, creates a high threshold voltage in the field effect transistor. When the control gate is grounded, current will not flow through the transistor, which is defined as a logic 0 state. Conversely, a reduction in the negative charge over the channel region creates a low threshold voltage, possibly negative. In this condition, with the control gate grounded, current will flow through the field effect transistor, which is defined as a logic 1 state.
One particular type of non-volatile memory device is the flash EEPROM (electrically-erasable-programmable-read-only-memory). Flash EEPROMs are a type of device which provide electrical erasing capability. The term "flash" refers to the ability to erase the memory cells simultaneously with electrical pulses. In an erased state, the threshold voltage of the field effect transistor is low and electrical current can flow through the transistor indicating a logic 1 state.
To program an EEPROM cell, typically, drain-side hot-electron injection is used to inject electrons onto either a floating gate electrode, or into trapping sites in a dielectric film overlying the channel region. The injection current can be enhanced by increasing either the channel electric field, or the electric field in the dielectric layer. To reduce the amount of time necessary to complete a programming operation, very high drain and gate voltages are used, such that the transistor is operating very close to breakdown during programming. However, the high voltages necessary for drain-side injection require that an additional power supply be provided to supply voltage levels in excess of the standard 5-volt operating voltage.
To reduce the current consumption necessary to program an EPROM cell, EEPROM devices have been developed which can be programmed and erased by Fowler-Nordheim tunneling. A thin tunnel oxide window is positioned in the gate dielectric near the source or drain region. To program the device, the gate is pulled to a high-voltage level, while the source, drain and substrate are held at ground potential. The high electric field generated across the thin tunnel oxide causes a tunneling current to flow, which charges the floating gate with electrons. Both hot carrier injection and Fowler-Nordheim programmable devices typically require a separate high-voltage power supply to provide voltage levels necessary for programming and erasing the cells.
A second type of memory device is known in the art as a SONOS (silicon-oxide-nitride-oxide-silicon) EEPROM device. The SONOS memory device consists of a single transistor which has a memory gate electrode disposed on an ONO (oxide-nitride-oxide) layer. A charge is stored in discrete traps in the bulk of the silicon nitride layer. The SONOS device is programmed by tunneling charge through a thin tunnel oxide layer from the substrate into the silicon nitride layer. Although SONOS devices offer the potential for low-voltage operation, further development is necessary to provide SONOS devices suitable for high-density integrated circuits.